1. Field of the Invention
The present invention relates to a multilayer golf ball which has a good durability to repeated impact with a golf club, a good feel when hit and improved distance.
2. Prior Art
A variety of multi-piece golf-ball constructions, including three-piece and four-piece balls, have been developed over the past few years in order to improve ball performance.
The practice is known of balancing a soft feel with good resilience in multi-piece golf balls by giving the ball a hardness distribution across its respective layers (core, intermediate layer and cover) in such a way as to retain both properties. In particular, a number of patents have been described on techniques for softening the core in order to achieve a soft feel (see, for example, JP-B 4-55077, JP 2674627, and JP-A 7-194735).
The cores of the golf balls disclosed in these patents all have a diameter of about 30 mm, or at least about 20 mm. Softening the core significantly lowers its resilience, which must then be compensated for by increasing the hardness of the intermediate layer and cover, to provide a reasonable resilience for the ball as a whole. However, increasing the hardness of these layers gives the ball a poor feel. In addition, stress concentration due to differences in hardness arises at the interface between the soft core and the hard intermediate layer, causing the layers to separate. JP-A 11-417 discloses a core provided with an inner layer of relatively small diameter. Yet, here too, there exists a large difference in hardness between the inner layer and the intermediate layer that has been formed around and encloses the inner layer, resulting in interfacial adhesion problems such as interlayer separation, and poor durability due to rubber fissuring in the intermediate layer.
It is therefore an object of the present invention to provide a solid, multilayer golf ball which has excellent durability to repeated impact with a golf club, a good feel when hit and increased distance.
The invention provides a solid golf ball comprising a solid core and a cover of at least one layer that encloses the solid core. The solid core has a multilayer construction which includes a center core and an outer core layer that encloses the center core. The center core is composed primarily of a thermoplastic resin or elastomer, and has a diameter of 3 to 18 mm and a Shore D hardness of 15 to 50. The outer core layer has a Shore D hardness near the interface thereof with the center core which is 1 to 15 units higher than the Shore D hardness of the center core.
In the solid golf ball of the present invention, the outer core layer is typically made of a rubber composition composed primarily of cis-1,4-polybutadiene and has a thickness of 1 to 16 mm. Preferably, at least one layer of the cover is harder than the outer core layer, and at least one layer of the cover is composed primarily of an ionomer resin. The center core typically has a specific gravity of 0.9 to 1.4.
The invention is directed to a solid golf ball comprising a solid core enclosed in a cover, wherein the solid core has both a center core and an outer core layer enclosing the center core. It has been found that by forming the center core to a small diameter, setting the Shore D hardness at the surface of the center core at from 15 to 50, and setting the Shore D hardness difference between the center core and the inside surface of the outer core layer within a specific range, the ball can be endowed with an improved durability to repeated impact with a golf club, a good feel and increased distance. It has also been found that using a resin material having a relatively low Shore D hardness in the center core greatly facilitates the grinding step, making it possible to efficiently produce small-diameter center cores. Moreover, the decline in the resilience due to the use of a resin material in the center core is relatively small for the ball as a whole because of the small diameter of the center core.